Radio frequency transmission waveguides



July 23, 1957 D. D. GRIEG ETAL 2,800,634

RADIO FREQUENCY TRANSMISSION WAVEGUIDES Original Filed June so, 1951 INVENTORS DONALD D. R/EG HERBfRT 55V tZ/MA/N inrj vz ATTORNEY United States Patent RADIO FREQUENCY TRANSMISSION WAVEGUID ES Donald D. Grieg, North Caldwell, and Herbert F. Engelmann, Mountain Lakes, N. J., assign'ors .to International Telephone and Telegraph Corporation, Nutley, N. 1., a corporation of Maryland Original application June 30, 1951, Serial No. 234,593, new Patent No. 2,721,312, dated Uctober 18, 1955. Divided and this application April 27, 1954, Serial No. 425,852

4 Claims. (Cl. 333-84) This invention relates to radio frequency transmission systems and more particularly to transmission wave-. guides for ultra high frequency signals. This is a division of our copending application Serial No. 234,503, filed June 30, 1951, now Patent No. 2,721,312.

In our copending application, Serial No. 227,896,, filed May 23, 1951, now abandoned, a new principle of waveguide transmission is disclosed, comprising in its simplest form two conductors, one as a ground conductor and the other as a line conductor, spaced close together in substantially parallel relation. The so-called ground conductor, which may be at ground potential or some other given potential, is considerably wider than the line conductor so that the surface thereof provides in effect an image reflection of the line conductor, whereby the dis tribution of the electric and magnetic fields between the conductors is substantially the same as the distribution between one conductor and the neutral plane of a theoretically perfect two-conductor parallel system. Small variations in size and shape of the line conductor may produce variations in the characteristic impedance of the system but the field distribution with respect to the ground conductor is not materially disturbed. Likewise, certain variations in the surface of the ground conductor do not materially disturb the field distribution with respect to the surface thereof since such variations either neutralize each other or do not adversely affect the field distribution between the two conductors. By this system, radio frequency waves can be easily propagated by a mode closely simulating the TEM mode along the line-ground conductor system.

One of the objects of this invention is to provide low loss transmission waveguide for use at ultra high frequencies.

One of the features of this invention is the provision of an air space between the two conductors in order to obtain a high Q waveguide. This is accomplished by supporting the line conductor on the face of a strip of dielectric disposed in spaced parallel relation to the ground conductor. The supporting dielectric strip is in turn supported by beads or narrow strips of dielectric located laterally of the line conductor.

The above-mentioned and other features and objects of this invention will become more apparent by reference to the following description taken in conjunction with the accompanying drawings, wherein:

Fig. 1 is a cross-sectional view of one form of transmission line in accordance with the principles of our copending application Serial No. 227,896;

Fig. 2 shows in cross-section one embodiment of the present invention; and

Figs. 3 and 4 are cross sectional views of additional embodiments of the invention.

Referring to Fig. 1 of the drawings, the radio frequency transmission waveguide illustrated is of the printed circuit type comprising a first or line condurtor 1 and a second or ground conductor 2 with a layer 3 of 2,800,634 Patented July 23, 1957 dielectric material therebetween. The conductive material may be applied to the layer of dielectric, such as polystyrene, polyethylene, 'Teflon (polytetrafluoroethylene) or other flexible insulation of high dielectric quality, in the form of conductive paint or ink, or the conductive material may be chemically deposited, sprayed through a stencil or dusted onto selected prepared surfaces of the dielectric according to known printed circuit techniques. For relatively short lengths of lines the conductive strips may be cut and applied by a die-stamping operation. In some cable manufacturing processes, the insulation may be extruded and simultaneously or later provided on the two sides thereof with conductive material of the desired thickness and widths. Where the widths of the two conductive layers are the same and it is desired to reduce the width of one of the layers, the portions of the two layers that are to be retained may be coated with a chemically inert material exposing the parts to be removed, and thereafter passing the cable through an etching bath, whereupon the exposed portions of the layers are removed.

While the two conductive layers 1 and 2 are shown in cross-section to be substantially rectangular, they may comprise different shapes so long as the ground conductor 2 presents a wide extended surface with respect to the line conductor. Preferably, the ground conductor should be from two to three times the width of the line conductor 1, although wider dimensions give still lower loss. In Fig. 1, the broken lines 4, 5 and 6 indicate, substantially the percentage of distribution of the electric field between the two conductors for a conductor relation wherein the ground conductor 2 is approximately three times the width of the line conductor, and the line conductor is wider than the spacing between conductors. The electric field concentrated within the lines 4 is from about to within the lines 5 it is from about to and within the lines 6 is is approximately 99%. From the foregoing it is clear that a narrow ground conductor may be used without much radiation loss, and where it can be three or more times the width of the line conductor an exceptionally low loss transmission line is assured.

In the embodiment of Fig. 2 the loss is minimized by replacing at least part of the dielectric between conductors by an air space. As shown, the transmission line comprises conductors 1 and 2 as in Fig. 1 but the dielectric is made up of three parts, namely, two side strips '7 and 8 and a cover strip 9. The two side strips 7 and 8 are formed on conductor 2 and secured thereto along the outer edge portions thereof. The upper layer 9 is preferably formed with conductor 1 embedded therein or coated thereon as indicated in Fig. 3. The upper strip is then applied to the other assembly either as shown in Fig. 2 with the conductor on the surface thereof opposed to the conductor 2, or if desired, the conductor 1 may be on the outer surface of the layer 9. In either case, an

air space 2i) is provided between the conductors 1 and 2 thereby maintaining the dielectric coeificient of the overall region bounded by the opposed surfaces of the conductors 1 and 2 at a value close to the dielectric coefficient of air.

In Fig. 4 a similar transmission line arrangement is provided with a relatively wide air space 11. In this form the conductor 1 is provided with an extended layer of dielectric 12 which corresponds to the width of the conductor 2. The conductor 2 is likewise provided with a dielectric layer 13. Interposed in the assembly at the outer edges thereof are two dielectric beads 14 and 15, which may be rectangular or other shape in cross-section, thereby insuring a wide air space 11. In this form the two dielectric layers 12 and 13 are interposed between the conductors 1 and 2. If desired, the upper layer 12 may be inverted so as to place the conductive coating 1 on the side thereof bounding the air space 11.

While We have described above the principles of our invention in connection with specific apparatus, it is to be clearly understood that this description is made by way of example only and not as' alimitation to the scope of our invention, as set forth in the objects thereof and in the accompanying claims.

We claim:

1. A transmission waveguide for propagating radio frequency energy comprising a first conductor, a second conductor, and means supporting said first and second conductors in closely spaced substantially parallel relation, the spacing between said first and second conductors being a small fraction of the wavelength of the mid-frequency of said radio frequency energy and the width of said second conductor being larger than the width of said first conductor to present thereto a planar surface for wave propagation in a mode simulating approximately the TEM mode with the main electromagnetic field distributed between the opposed surfaces of said conductors, said supporting means being disposed outside the main field region leaving solely an air gap separating said opposed surfaces, said gap being substantially Wider than said first conductor.

2. A transmission waveguide for propagating radio fre-,

quency energy comprising a first conductor, a second conductor, and means supporting said first and second conductors in closely spaced substantially parallel relation, the width of said second conductor being greater than the width of said first conductor to present thereto a planar conducting surface for wave propagation in a mode simulating approximately the T EM mode with the main electromagnetic field distributed between the opposed surfaces of said conductors, and said means being of dielectric material in a form to provide solely an air space having a width substantially greater than the width of said first conductor disposed directly between the opposed surfaces of said conductors and throughout the main field region.

3. A transmission waveguide for propagating radio frequency energy comprising a strip of dielectric material carrying a first strip of conductive material of a given width, a second strip of conductive material facing said first strip of conductive material and of a width sufficient- 1y greater than said given width as to present to said first conductive strip a planar conducting surface, and means supporting said dielectric strip in closely spaced parallel relation to the planar surface of said second conductive strip to provide solely a dielectric air space between the opposed surfaces of said strips of conductive material and to the sides thereof to provide a mode of propagation in which the main electromagnetic field is distributed between the opposed surfaces of said conductor in said air space, with said supporting means being disposed outside of said main field region.

4. A transmission waveguide for propagating radio fre quency energy according to claim 3 wherein said strip of dielectric material extends laterally with respect to the side edges of said first conductive strip, and said supporting means includes a pair of narrow dielectric strips disposed laterally of said first conductive strip in supporting engagement between said first mentioned dielectric strip and said second conductive strip.

References Cited in the file of this patent UNITED STATES PATENTS 2,159,648 Alford May 23, 1939 2,409,449 Sanders Oct. 15, 1946 2,611,822 Bliss Sept. 23, 1952 

